#include<bits/stdc++.h>
using namespace std;
typedef pair<int,int> pii;
const double I=1e8;
inline int f(int x){return x<0?1e9:x;}
int main(){
  freopen("talk.in","r",stdin);
  freopen("talk.out","w",stdout);
  ios::sync_with_stdio(false);
  cin.tie(0); cout.tie(0);
  int n,m,x=0; cin>>n>>m;
  vector<pii> a(n);
  for(auto &i:a)cin>>i.first>>i.second;
  sort(a.begin(),a.end(),[&](pii x,pii y){
    if(x.second==y.second)return x.first<y.first;
    return f(x.second)<f(y.second);
  });
  vector<vector<double> > f(n+1,vector<double>(n+1,I));
  f[0][0]=0;
  for(auto e:a){
    int x,y; auto g=f; tie(x,y)=e;
    for(int i=n-1;~i;i--)
      for(int j=n-1;~j;j--){
        g[i+1][j]=min(g[i+1][j],f[i][j]+1.0*x/(j+1));
        g[i+1][j+1]=min(g[i+1][j+1],f[i][j]+1.0*y/(j+1));
      }
    f=g;
  }
  cout<<fixed<<setprecision(9)<<*min_element(f[m].begin(),f[m].end())<<endl;
  return 0;
}